During initial studies to investigate changes in individual bile acid concentrations in serum in rats with induced disruptions in the enterchepatic circulation, an unidentified bile acid that is an insignificant form in normal rats became one of the predominant bile acids during cholestasis. This unidentified form increased in animals with chemical (a-naphthylisothiocyanate: ANIT) and physical impairment of bile flow. This year the unidentified bile acid was isolated and collected using HPLC separation. Preliminary NMR studies indicate that the compound has the nuclear structure of b-muricholic acid (3a, 6b, 7b-trihydroxy-5b-cholanoic acid) and is unsaturated at C22 which is in the 5 carbon chain attached to C17. Upon confirmation of this structure. We will have identified a novel bile acid in rats that may be pathognomonic for cholestasis. Investigations during the next year will explore biochemical mechanisms responsible for the synthesis of this bile acid and conditions responsible for their induction. We recently reported that bile acid profiles in the serum of rats treated with a peroxisome proliferator were altered in several important respects. In addition to the finding that cholic acid concentrations were significantly increased while those of chenodeoxycholic acid were significantly decreased, we also noted that concentrations of glycine conjugated bile acids were increased and those of taurine conjugated bile acids were decreased. This has not been previously reported. We are using a liver slice culture system that we have established in our laboratory to examine mechanisms of bile acid conjugation in liver from normal animals and from animals treated with a peroxisome proliferator. To elucidate the mechanism(s) responsible for changes in conjugation of bile acids, concentrations of glycine and taurine in the liver samples in addition to characterization of kinetics of the conjugating enzymes in the different organelles are being determined. Additionally, a model of chronic liver disease will be used to explore mechanisms for the therapeutic effect of urosdeoxycholate.